1. Field of the Invention
The present invention relates generally to gas purification and more particularly to gas purifiers containing dispersed impurity-sorbing materials.
2. Description of the Related Art
Ultra-high purity (UHP) gases are used for the manufacture of semiconductor devices, laboratory research, mass spectrometer instruments and other industries and applications. UHP gases are typically defined as at least 99.9999999% pure gas by volume. There are several methods of producing UHP gases. Purifiers are widely used based on the use of solid materials that can bond impurities in the stream of a main gas, by interacting with the impurity molecules according to a variety of mechanisms.
An important class of gas purifiers exploits the properties of getter alloys, which include Zr, Ti, Nb, Ta, and V based alloys as active elements. Examples of commonly used alloys are an alloy of weight percent composition Zr 70%-V 24.6%-Fe 5.4%, under the trademark St 707; an alloy of composition Zr 76.5%-Fe 23.5%, under the trademark St 198; an alloy of composition Zr 84%-Al 16%, under the trademark St 101; and certain Tixe2x80x94Ni alloys, all of which are produced and sold in conjunction with gas purifiers by SAES Pure Gas, Inc. of San Luis Obispo, Calif.
The working principle of getter alloys is chemisorption of species such as O2, H2O, CO, CO2 and CH4, through surface adsorption followed by dissociation and diffusion in the bulk of the getter material of the atoms making up the impurity molecules. Some getter alloys may also fix N2 according to the same mechanism. The result is the formation of oxides, carbides or nitrides of the metals of the alloy. Because the species formed are very stable, the sorption of the above mentioned gases by getter alloys is essentially irreversible.
Because getter alloys do not react with noble or inert gases, they are well suited for purification of these gases. By using these alloys it is possible to remove traces of reactive gases from inert gases. Examples of gases that may be purified by means of getter alloys include noble gases, chloroflourocarbons, which are used in the semiconductor industry, and nitrogen N2). For example, N2 may be purified by the St 198 alloy, which has a negligible sorption capability for the gas. Examples of purifiers based on the use of getter alloys are disclosed in UK Patents GB 2,177,079 and GB 2,177,080, in European Patent EP 365490, and in U.S. Pat. No. 5,194,233 and 5,294,422.
FIG. 1A is a schematic illustration of a getter purifier 10 of the prior art during process gas purification at an elevated temperature. Getter purifier 10 includes a chamber 12, which is coupled to an inlet 14 and an outlet 16. Chamber 12 is partially filled with getter material particles 18. A heater 20 heats getter purifier 10 to at least about 300 degrees Celsius. A process gas with gaseous impurities such as water or carbon oxide is introduced into chamber 12 through inlet 14 where getter material particles 18 absorb the traces of water and carbon oxide. A purified process gas then exits chamber 12 through outlet 16.
While getter materials show essentially irreversible gettering for impurities (e.g. oxygen, water, carbon monoxide, carbon dioxide, methane) normally present in noble or relatively inert gases (such as argon, helium and nitrogen) for semiconductor industry, getter materials behave very differently towards hydrogen. In fact, getter materials show reversible gettering for hydrogen, which undergoes an equilibrium reaction with most getter materials. At about room temperature, the pressure of xe2x80x9cfreexe2x80x9d gas at is very low, but the pressure increases with increasing temperature.
FIG. 1B is a schematic illustration of a getter purifier 10 of the prior art during the removal of hydrogen from a process gas. Getter purifier 10 is operational at ambient temperatures (0 to 40 degrees Celsius) to remove traces of hydrogen from process gases. If a process gas with hydrogen is introduced into chamber 12 through inlet 14, getter material particles 18 will absorb the hydrogen, leaving a purified process gas to exit chamber 12 through outlet 16.
Getter based purifiers are highly efficient in removing impurities as shown in FIG. 1A, but they are costly and need to be kept at about 300 to about 450xc2x0 C. for operation. Therefore, in some circumstances other kinds of purifiers are preferred. An example of lower cost purifiers is the so-called nickel purifiers, which operate at around room temperature. These purifiers include as the active material, metallic nickel, generally supported on a porous substrate such as silica.
FIG. 2A is a schematic illustration of a nickel purifier 22 of the prior art during process gas purification. Nickel purifier 22 includes a chamber 24, which is coupled to an inlet 26 and an outlet 28. Chamber 24 is partially filled with nickel material particles 30. Nickel is typically present in metallic form for at least 5% of the overall amount of nickel material particles 30, with the remainder generally being present as nickel oxide, NiO. Nickel is generally present in a particulate or xe2x80x9cdispersedxe2x80x9d form, so as to have a high specific area of at least 100 m2/g and preferably between about 100 and 200 m2/g, but the overall amount of nickel is limited. By xe2x80x9cdispersedxe2x80x9d it is meant that the material is formed by discrete particles, such as powders, granules, pellets, etc.
Nickel purifiers often also contain physical water sorbers, such as molecular sieves, to help remove water vapor and leave nickel material available for removal of oxygen and carbon oxides. As shown, a process gas, water, and trace amounts of oxygen and carbon oxide enter chamber 24 through inlet 26. During operation of nickel purifier 10, nickel material particles 18 react with oxygen or water and with CO or CO2. The product of the Ni and oxygen or water reaction is NiO. Once the sorbing capacity of nickel material particles 18 has reached its limits, the purifier may be regenerated.
FIG. 2B is a schematic illustration of a nickel purifier 22 of the prior art during the process of regeneration. Nickel material particles 30 are regenerated by passing a flow of hydrogen-containing inert gas over the nickel material particles 30 maintained at a temperature of about 200xc2x0 C. by heater 20. The inert gas is preferably nitrogen, the amount of hydrogen is preferably below about 20% by volume, and more preferably between about 2 and about 5% by volume of the flowing gas, and the regeneration process is preferably continued for about 14-20 hours. In these conditions NiO and the product of the reaction of Ni and CO/CO2 are reduced to metallic nickel. Nickel purifiers are disclosed, e.g., in U.S. Pat. No. 4,713,224.
Because water and CO are produced during the regeneration step, the operation must be performed with the purifier disconnected from the pure gas line, in order not to pollute the system. A wide range of nickel-based purifiers is sold by Aeronex Inc. of San Diego, Calif. under the name GATEKEEPER(copyright). Further to the application indicated above, another important use of nickel-based purifiers is in gas cabinets, for the purification of gas (generally nitrogen) used to purge gas pipelines during process gas cylinders change out.
FIG. 3 illustrates another nickel purifier unit 32 of the prior art. Nickel purifier unit 32 includes a body or enclosure 33 defining a chamber 34, which is generally made of stainless steel into an essentially cylindrical shape. Chamber 34 is preferably electropolished to at least 10 Ra. At the two opposing bases of nickel purifier unit 32, a gas inlet 36 and an outlet opening 38 are provided. Gas inlet 36 and outlet opening 38 are typically equipped with suitable fittings 40 for connection to a set of gas lines. Fittings 40 shown are male face seal fittings, but as is well known in the art, compression fittings may also be used. Nickel purifier unit 32 is preferably equipped with particle filters at gas inlet 36 and outlet opening 38. Particle filters are generally made of sintered stainless steel particles and capable of retaining particles of dimensions of 0.003 xcexcm and larger.
The internal volume of nickel purifier 32 is filled with particles of nickel-containing or nickel supporting materials. These materials may be made of formed pieces (spheres or cylinders) of a porous supporting medium, such as silica, over which nickel material is dispersed according to techniques well-known in the field of catalysts production. Nickel may be present in a mixed form, in which part of the metal is present as a compound, generally nickel oxide, NiO, with at least 5% of the metal present in reduced metallic form.
A major disadvantage of nickel-based purifiers is that regeneration is not easily accomplished on site, due to the need of keeping for hours the purifier under a hydrogen-containing gas flow that, at the outlet, need be vented outside the system; as a consequence, for the regeneration operation the purifier must generally be returned to the manufacturer. To avoid service interruptions, producers generally offer systems made up of two nickel purifiers in parallel, so that one can operate while the other is regenerated.
Also well known are purifiers where both getter and nickel beds are used. These purifiers are disclosed, e.g. in U.S. Pat. Nos. 5,492,682, 5,558,844, 5,556,603 and 5,902,561. These patents show two-stage purifiers, in which the gas first contacts a bed of nickel material kept at room temperature and then a second bed of getter material maintained at a temperature of between about 250 to about 400xc2x0 C. In these purifiers each bed works according to its normal operation as described before.
In view of the foregoing, it is desirable to have a method and apparatus for efficiently and economically rejuvenating a gas purifier, particularly so that it is possible to rejuvenate the purifier on site.
The present invention provides a method and apparatus to purify various gases utilizing a gas purifier capable of operating at room temperature, but such that can easily be rejuvenated at the point of use when saturated by simply isolating it from the gas line it is inserted in and heating the apparatus at a pre-set temperature. It should be appreciated that the present invention can be implemented in numerous ways, including as a process, an apparatus, a system, a device or a method. Several inventive embodiments of the present invention are described below.
In one embodiment of the present invention, a rejuvenable ambient temperature purifier is provided. The purifier includes an enclosure with a chamber having an inlet and an outlet. Purifier material is disposed within the chamber. The transition metal material is preferably in a dispersed form with preferably at least 5% of the transition metal material being in a metallic form. The getter material is also preferably in a dispersed form intermixed with the transition metal material. The getter material is preferably selected from the group including Zr, Ti, Nb, Ta, V, and alloys thereof.
In another embodiment of the present invention, a rejuvenable ambient temperature purifier system is provided. The system comprises a purifier including an enclosure with a chamber having an inlet and an outlet. Purifier material comprising a mixture of a transition metal material and a getter material is disposed within the chamber. The transition metal material is preferably in a dispersed form with preferably at least 5% of the transition metal material being in metallic form. The getter material is also preferably in a dispersed form intermixed with the transition metal material. The getter material is selected from the group including Zr, Ti, Nb, Ta, V, and alloys thereof. The purifier system also includes an inlet valve coupled to the inlet and an outlet valve coupled to the outlet. A heater is associated with the purifier for heating the purifier to at least about 200 degrees Celsius.
In yet another embodiment of the present invention, a method for rejuvenating an ambient temperature purifier having a mixture of transition metal material and getter material is provided. The method includes sealing a purifier in a working environment. A mixture of a transition metal material and a getter material are disposed within the purifier chamber. The transition metal material is preferably in a dispersed form with preferably at least 5% of the transition metal material being in metallic form. The getter material is also preferably in a dispersed form intermixed with the transition metal material. The getter material is preferably selected from the group including Zr, Ti, Nb, Ta, V, and alloys thereof. The purifier is heated to at least about 200 degrees Celsius, and then cooled so that the purifier can achieve a substantially ambient temperature of its working environment. Finally, the purifier is unsealed, and ready to be used again.
In yet another embodiment of the present invention, a method for purifying gases at ambient temperatures is provided. The method includes providing a purifier having a sealable enclosure. The enclosure defines a chamber having an inlet and an outlet. A mixture of a transition metal material and a getter material are disposed within the purifier chamber. The transition metal material is preferably in a dispersed form with preferably at least 5% of the transition metal material being in metallic form. The getter material is also preferably in a dispersed form intermixed with the transition metal material. The getter material is preferably selected from the group including Zr, Ti, Nb, Ta, V, and alloys thereof. Gases flowing into the inlet are purified through the purifier material. Gas then flows out of the outlet at about ambient temperatures, whereby the transition metal material adsorbs water, oxygen and carbon monoxide and the getter material adsorbs hydrogen. The inlet and outlet are then closed to seal the enclosure.
The purifier is heated to at least 200 degrees Celsius, whereby the getter material releases hydrogen. The hydrogen removes oxygen and carbon from the transition metal material. Excess hydrogen is then adsorbed by the getter material. The purifier is then cooled so that the purifier can return to about ambient temperature of its working environment. Finally, the purifier is unsealed by opening the inlet and outlet, rejuvenated for the purification of gases.